Building and removing stratification in the Arctic Ocean

Transcription

1 Building and removing stratification in the Arctic Ocean John Marshall Massachusetts Institute of Technology With help and advice from: An Nguyen Patrick Heimbach Hajoon Song Christopher Klingshirn FAMOS School for young scientists Tuesday, October 22 nd, 2013

2 Water mass transformation Buoyancy loss Suck Buoyancy gain Pump eddies heavy subduction light subduction Briefly review aspects of circulation in the Arctic basin Dynamics of the Beaufort Gyre Example of light subduction

3 Water mass transformation heavy subduction preconditioning cool, suck Greenland and Labrador Sea mixing spreading

4 Dynamical ideas Extract buoyancy from surface of homogeneous, rotating ocean Jones and Marshall, 1993 H Numbers Helfrich 1994 f 10 4 s 1 Ocean - rotation important Natural Rossby number R o l rot H Radius of deformation 1 H B f B 10 7 m 2 s 3 H 1km l rot 1km R o Atmosphere - rotation not important on convective scale l H R o B 10 2 m 2 s 3 H 10km l rot 100km R o 10 50

5 Interplay between convection and baroclinic instability cool Vertical velocity w Upright convection Jack Whitehead Convection Baroclinic instability reminiscent of metrological flows Baroclinic instability Eddies flux buoyancy vertically to offset loss from the surface

6 Light subduction Subtropical gyres!

7 Building Blocks f-plane Cool, suck -plane gyres Cool, suck Warm, pump Warm, pump Warm, pump Antarctic Circumpolar Current Cool, suck

8 Review aspects of Arctic Circulation Beaufort Gyre

9 Simulation of the Arctic Ocean using MITgcm ECCO2 Project: collaboration between MIT and JPL

10 ECCO2 5-year climatology ( ) T at 200m 400m 500km

11 km Blue circulation is sensitive to local atmospheric circulation Anticyclonic circulation regime: JGR, 1997 Proshutinsky and Johnson

12 Wind stress Mean wind stress N/m**2 Net freshwater flux in to the Arctic Balanced by ice and freshwater export

13 Section through the Beaufort Gyre Pumping down of fresh water by the wind Fresh water lens T colored S contoured Halocline layer Light subduction Vast stores of available potential energy in the fresh water lens. Expect vigorous baroclinic instability.

14 Arctic is seething with baroclinic instability and geostrophic turbulence

15 Dynamics of the Beaufort Gyre What sets depth and stratification of the freshwater lens? Downward buoyancy flux could be balanced by small-scale mixing or eddy fluxes. Note cannot use classic thermocline theory 0

16 Laboratory experiment: f -plane warm pumped lenses Marshall et al. (2002) T t T profile under disc N 2

17 Theory Q r o 2 w Ek eddies fw Ek g r o

18 Numbers for freshwater lens e-folding scale h fw g r h 4 s 1 10 m/ y 500km m s 2 40m 500km l rot wg f m R o l rot h 1 L 4km l rot r In the correct ball-park

19 e-folding scale of 70m S T Data courtesy of Mary-Louise Timmermans Ice-Tethered Profilers John Toole

20 Conclusions Dynamical and water mass transformation processes in the Arctic basin are complex: --- observations reveal extraordinary detail --- models (GCMs) can capture only broad aspects Geostrophic turbulence is ubiquitous ---- not just noise, surely there for a reason Beaufort gyre is a beautiful example of Light Subduction Geostrophic eddies likely play a role in equilibrating the fresh water lens Flow of Atlantic water at depth likely has very different dynamics See, e.g., Nost and Isachsen, JMR, 2003